In the usual weather resistance and solar radiation resistance tester, a specimen supported on a specimen rotating frame is exposed to light energy under suitably selected temperature and relative humidity conditions as it is revolved around a light source such as a carbon arc lamp or a xenon lamp disposed at the center of the rotating frame for testing the specimen for light fastness and weather resistance. In order to be able to measure or adjust the light energy incident on the sample, it has been the usual practice to provide a light receiving element such as a photoelectric tube or photomal (photoelectric multiplier tube) on the surface of the specimen and connect it to a meter outside the tester. In such an arrangement, leads from the light receiving element are led through a pipe constituting the shaft of the sample rotating frame and are connected at the lower end to contact means constituted by a slip ring for connection to the meter so that they are not twisted during the rotation of the rotating frame.
The prior art contact means of this type has a structure consisting of a disc or a rotor of a similar shape made of an insulating material, on which a metal ring and metal or carbonaceous slide contacts are provided, and the contact portion thereof is the so-called dry type, that is, it is exposed to air.
The light receiving section consisting of the photoelectric tube or photomal has a high input impedance (i.e. input resistance) which is required for its performance, and even a slight reduction of the insulation resistance in the contact section leads to errors in the measurement. Therefore, the prior art contact means which is exposed to air must always be kept clean because contamination of the insulating material between the two electrodes or between an electrode and ground due to contact wear causes a reduction of the insulation resistance. For example, a resistance which is greater than 2,000 M.omega. when the contact means is new is sometimes reduced to about 20 M.omega. due to contamination. In this case, current flows in the photoelectric tube even if it does not receive light energy, that is, the tube indicates an apparent light reception condition. Also, since the contact means is exposed to air, the resistance thereof is likely to be reduced due to the effect of moisture.